Invited ReviewZoonotic potential of Giardia
Graphical abstract
Introduction
Species of the genus Giardia infect numerous hosts, ranging from mammals to amphibians and birds. It is one of the most common intestinal parasites of humans; approximately 200 million people in Asia, Africa and Latin America have symptomatic infections (World Health Organisation, 1996, Yason and Rivera, 2007). Clinical manifestations of giardiasis are quite variable and range from the absence of symptoms to acute or chronic diarrhoea, dehydration, abdominal pain, nausea, vomiting and weight loss (Eckmann, 2003, Cacciò and Ryan, 2008). Prevalences of giardiasis in humans are generally lower in developed countries with 0.4–7.5% reported for developed countries and 8–30% for developing countries (Feng and Xiao, 2011).
The life cycle of Giardia is direct and involves just two major stages, the trophozoite, which is the replicative stage, and the cyst, which is the infective stage. Infection is initiated either by consumption of contaminated food or water or by the faecal-oral route via person-to person or animal-to-animal contact. Exposure to the acidic environment of the stomach provides the necessary stimuli for the excystation of the trophozoite from the cyst in the duodenum of the small intestine (Gardner and Hill, 2001). Trophozoites undergo repeated mitotic division and are eventually triggered to form environmentally resistant cysts in response to the bile conditions of the small intestine, which are then shed in faecal material. Cysts are immediately infectious when excreted in faeces, are remarkably stable and can survive for weeks to months in the environment. As a result of this, environmental contamination can lead to the contamination of drinking water and food (Feng and Xiao, 2011). In humans, the infective dose is approximately 10–100 cysts (Rendtorff, 1954). Most outbreaks of giardiasis have been linked to the consumption of contaminated drinking water. In a recent review it was reported that of the 199 published outbreaks caused by protozoa during the period 2004–2010, 70 (35%) were caused by Giardia (Baldursson and Karanis, 2011).
Section snippets
Taxonomy
Currently, six Giardia spp. are accepted by most researchers on the basis of the morphology of trophozoites and/or cysts. This comprises Giardia agilis in amphibians, Giardia ardeae and Giardia psittaci in birds, Giardia microti and Giardia muris in rodents, and Giardia duodenalis in mammals. Another species, Giardia varani, has been described from a water monitor (Varanus salvator) (Upton and Zien, 1997). This parasite lacks median bodies and had binucleated cysts but its identity has not been
Molecular tools for genetic characterization of Giardia
The introduction of molecular techniques, in particular those based on the in vitro amplification of nucleic acids (i.e., PCR and related methodologies), has revolutionised the study of the epidemiology of giardiasis. Molecular tools are thought to provide higher sensitivity and specificity compared with both microscopic or immunological assays, and offer the possibility to identify Giardia isolates at the level of species, assemblage, sub-assemblage and genotype (Cacciò and Ryan, 2008).
The
Molecular epidemiology of giardiasis in humans
Humans are infected by two G. duodenalis assemblages, namely assemblages A and B (Mayrhofer et al., 1995). Molecular analysis of more than 2,800 samples (Table 3) indicates that assemblage B (∼58%) has a higher prevalence than assemblage A (∼37%) worldwide. This proportion does not change when data from either developed or developing countries are analysed. However, the prevalence of mixed infections is higher (5.2%) in developing countries than in developed ones (3.2%). It should be noted that
Zoonotic potential of Giardia
Animals can harbour both zoonotic and host-specific G. duodenalis assemblages that are morphologically identical and therefore sensitive typing tools are required to track transmission (Feng and Xiao, 2011). Unfortunately, tools that allow sensitive amplification of genetic loci from clinical samples and that provide sufficient genetic discrimination to infer origins or sources of transmission for Giardia are yet to be developed. When using molecular analysis to identify assemblages in animal
Perspectives
The use of molecular epidemiological tools, and particularly sub-typing tools, has greatly changed our understanding of zoonotic transmission of Giardia spp. Genetic typing supports the zoonotic potential of Giardia cysts shed by animals. Sub-assemblages AI and AII are found in both humans and animals but sub-assemblage AI is preferentially found in livestock and pets whereas sub-assemblage AII is predominantly found in humans. Sub-assemblage AIII is almost exclusively found in wild ungulates
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